Computer Networks

1. Computer Networks Lecture 2 Adrian Sergiu DARABANT

2. Little Endian/Big endian • In memory data representation • Big endian – most significant byte first • Little endian – least significant byte first 46F4 – Little end. => F4 46 - Big end. => 46 F4

3. Little Endian/Big endian

4. Float and double Float – 4 bytes exp mantisa Float = (sign?-1:1) * 2^exp *1.<mantisa> Double - 8 bytes Same endianness as integers Swapping 8 or more byte entities ?!?

5. Advanced TCP – I/O Modes • Blocking I/O • Nonblocking I/O • I/O multiplexing (select and poll) • Signal driven I/O (SIGIO) • Asynchronous I/O (the POSIX aio_functions)

6. Network operation steps • Waiting for the data to be ready – i.e. data arrives from the network • Copying the data from the kernel to the process – i.e. copying the data from the kernel buffer into the application space.

7. Blocking I/O Model

8. Non-Blocking I/O Model

9. Multiplexed I/O Model Involves using select and poll

10. Signal I/O Model

11. Asynchronous I/O Model • aio_read – asynchronous POSIX read • Async - Kernel tells us when the operation is complete • Signal – Kernel tells us when the operation can start !!

12. Blocking I/O Operations Sequence Client Server Write(…) Write(…) Read(…) Read(…) Read(…) Write(…) ? Write(…) Write(…) Read(…) Write(…) Read(…) Write(…) ? Read(…) Write(…) Read(…) Read(…) Read(…) Write(…) ?

13. TCP • Connection-Oriented • Recv • Send • Guaranteed data delivery • Guaranteed data ordering delivery • Type = SOCK_STREAM – when creating socket

14. UDP • Connection-Less – datagram oriented • Recvfrom • Sendto • No guarantee for datagram delivery • No guarantee for datagram ordering • Type = SOCK_DGRAM – when creating socket

15. The select system call #include <sys/select.h> #include <sys/time.h> int select(int maxfd+1, fd_set *readset, fd_set *writeset, fd_set *exceptset, const struct timeval *timeout); Returns: • positive count of ready descriptors • 0 if timeout • -1 on error • void FD_ZERO(fd_set *fdset); - clear all bits in fdset • void FD_SET(int fd, fd_set *fdset); - turn on the bit for fd in fdset • void FD_CLR(int fd, fd_set *fdset); - turn off the bit for fd in fdset • int FD_ISSET(int fd, fd_set *fdset); - IS the fd ready ? • BE WARNED – select modifies readset, writeset and exceptset

16. Conditions for a socket to be ready

17. Client-Server TCP/IP Apps • Server listens for client’s requests, executes them and answers. • Server types (from the Comm Point of View) • Iterative Servers (blocking) • Concurrent servers (fork, threads) • Concurrent multiplexed servers. (select)

18. Working with the DNS struct hostent *gethostbyname(const char *name); struct hostent *gethostbyaddr(const void *addr, int len, int type); hostent structure is defined in <netdb.h> as follows: struct hostent { char *h_name; /* official name of host */ char **h_aliases; /* alias list */ int h_addrtype; /* host address type */ int h_length; /* length of address */ char **h_addr_list; /* list of addresses */ } #define h_addr h_addr_list[0] ------------------------------------------------------------------------------- Another Approach: getaddrinfo(….), getnameinfo(….)

19. MultiClient Chat Server -1 int fdmax; // maximum file desc. number int listener; // listening socket descriptor int newfd; // newly accept()ed socket char buf[256], tmpbuf[256]; int nbytes, ret; int yes=1; // setsockopt() SO_REUSEADDR int addrlen; int i, j, crt, int_port,client_count=0; #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> fd_set master; // master file descriptor list fd_set read_fds; // temp file descriptor list for select() struct sockaddr_in myaddr; // server address struct sockaddr_in remoteaddr;

20. MultiClient Chat Server -2 struct sockaddr_in getSocketName(int s, bool local_or_remote) { struct sockaddr_in addr; int addrlen = sizeof(addr); int ret; memset(&addr, 0, sizeof(addr)); ret = (local_or_remote==true?getsockname(s,(struct sockaddr *)&addr, (socklen_t*)&addrlen): getpeername(s,(struct sockaddr *)&addr, (socklen_t*)&addrlen) ); if (ret < 0) perror("getsock(peer)name"); return addr; }

21. MultiClient Chat Server -3 char * getIPAddress(int s, bool local_or_remote) { struct sockaddr_in addr; addr = getSocketName(s, local_or_remote); return inet_ntoa(addr.sin_addr); } int getPort(int s, bool local_or_remote) { struct sockaddr_in addr; addr = getSocketName(s, local_or_remote); return addr.sin_port; }

22. MultiClient Chat Server -4 // send to everyone void sendToALL(char * buf, int nbytes) { int j, ret; for(j = 0; j <= fdmax; j++) { if (FD_ISSET(j, &master)) // except the listener and ourselves if (j != listener && j != crt) if ( send(j, buf, nbytes, 0) == -1) perror("send"); } return; }

23. MultiClient Chat Server -5 int main(int argc, char **argv) { if (argc < 2 ) { printf("Usage:\n%s <portno>\n",argv[0]); exit(1); } int_port = atoi(argv[1]); FD_ZERO(&master); // clear the master and temp sets FD_ZERO(&read_fds); // get the listener if ((listener = socket(AF_INET, SOCK_STREAM, 0)) == -1) { perror("socket"); exit(1); } // get rid of the "address already in use" error message if (setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int) ) == -1) { perror("setsockopt:"); exit(1); }

24. MultiClient Chat Server -6 // bind memset(&myaddr, 0, sizeof(myaddr)); myaddr.sin_family = AF_INET; myaddr.sin_addr.s_addr = INADDR_ANY; myaddr.sin_port = htons(int_port); if (bind(listener, (struct sockaddr *)&myaddr, sizeof(myaddr)) == -1) { perror("bind:"); exit(1); } // listen if (listen(listener, 10) == -1) { perror("listen"); exit(1); } // add the listener to the master set FD_SET(listener, &master); // keep track of the biggest file descriptor fdmax = listener; // so far, it's this one

25. MultiClient Chat Server -7 // main loop for(;;) { read_fds = master; // copy it – select if (select(fdmax+1, &read_fds, NULL, NULL, NULL) == -1) { perror("select"); exit(1); } // run through the existing connections looking for data to read for(i = 0; i <= fdmax; i++) { if (FD_ISSET(i, &read_fds)) { // we got one!! crt = i; if (i == listener) { // handle new connections addrlen = sizeof(remoteaddr); if ((newfd = accept(listener, (struct sockaddr *)&remoteaddr,(socklen_t*)& addrlen)) == -1) { perror("accept"); }

26. MultiClient Chat Server -8 else { FD_SET(newfd, &master); // add to master set if (newfd > fdmax) { // keep track of the maximum fdmax = newfd; } printf("selectserver: new connection from %s on socket %d\n", getIPAddress(newfd, false),newfd); client_count++; sprintf(buf,"Hi-you are client :[%d] (%s:%d) connected to server %s\nThere are %d clients connected\n", newfd, getIPAddress(newfd,false), getPort(newfd, false), getIPAddress(listener, true), client_count); send(newfd,buf,strlen(buf)+1,0); } }

27. MultiClient Chat Server -9 else { // handle data from a client if ((nbytes = recv(i, buf, sizeof(buf), 0)) <= 0) { // got error or connection closed by client if (nbytes == 0) { // connection closed printf("<selectserver>: client %d forcibly hung up\n", i); } else perror("recv"); client_count--; close(i); // bye! FD_CLR(i, &master); // remove from master set } else { // we got some data from a client - check for connection close request buf[nbytes]=0; if ( (strncasecmp("QUIT\n",buf,4) == 0)) { sprintf(buf,"Request granted [%d] - %s. Disconnecting...\n",i,getIPAddress(i,false));

28. MultiClient Chat Server -10 send(i,buf, strlen(buf)+1,0); nbytes = sprintf(tmpbuf,"<%s - [%d]> disconnected\n",getIPAddress(i,false), i); sendToALL(tmpbuf,nbytes); client_count--; close(i); FD_CLR(i,&master); } else { nbytes = sprintf(tmpbuf, "<%s - [%d]> %s",getIPAddress(crt, false),crt, buf); sendToALL(tmpbuf, nbytes); } } } } } } return 0; }

29. MultiClient Chat Client -1 ….//include stuff ….. fd_set read_fds,master; // temp file descriptor list for select() int sock; //socket struct sockaddr_in servaddr; char buf[256]; // buffer for client data int nbytes, ret, int_port; int main(int argc, char **argv) { if (argc < 3 ) { printf("Usage:\n%s <hostname or IP address> <portno>\n",argv[0]); exit(1); }

30. MultiClient Chat Client -2 int_port = atoi(argv[2]); int ipaddr = inet_addr(argv[1]); // check if address is a hostname if (ipaddr == -1 ) { struct in_addr inaddr; struct hostent * host = gethostbyname( argv[1] ); if (host == NULL ) { printf("Error getting the host address\n"); exit(1); } memcpy(&inaddr.s_addr, host->h_addr_list[0],sizeof(inaddr)); printf("Connecting to %s ...\n",inet_ntoa( inaddr) ); memcpy(&ipaddr, host->h_addr_list[0],sizeof(unsigned long int)) ; } // create the socket if ((sock = socket(AF_INET, SOCK_STREAM, 0)) == -1) { perror("socket"); exit(1); }

31. MultiClient Chat Client -3 memset(&servaddr,0, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = ipaddr; servaddr.sin_port = htons( int_port ); // connect to server if (connect(sock, (struct sockaddr *)&servaddr, sizeof(servaddr)) < 0 ) { perror("connect"); exit(1); } // add the socket to the master set FD_ZERO(&read_fds); // clear the set FD_ZERO(&master); FD_SET(0, &master); FD_SET(sock, &master); while(1) { read_fds = master; if (select(sock+1, &read_fds, NULL, NULL, NULL) == -1) { perror("select"); exit(1); }

32. MultiClient Chat Client -4 if ( FD_ISSET(0, &read_fds) ) { // check if read from keyboard nbytes = read(0, buf,sizeof(buf)-1); ret = send(sock, buf, nbytes,0); if (ret <= 0 ){ perror("send"); exit(1); } } if ( FD_ISSET(sock, &read_fds) ) { // check if read from server nbytes = read(sock, buf, sizeof(buf)-1); if (nbytes <= 0) { printf("Server has closed connection... closing...\n"); exit(2); } write(1,buf, nbytes); } } return 0; }

33. Socket Options setsockopt(int s, int level, int optname,void *optval, socklen_t *optlen) getsockopt(…..) Optname SO_REUSEADDR – reuse local addresses SO_BROADCAST – enables broadcast

34. The OSI Reference Model All People Seem To Need Data Processing

35. Principles of the OSI model • A layer should be created where a different abstraction is needed. • Each layer should perform a well-defined function. • The function of each layer should be chosen with an eye toward defining internationally standardized protocols. • The layer boundaries should be chosen to minimize the information flow across the interfaces. • The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy.

36. The Physical Layer • Raw bits over a communication channel • Data representation • 1–how many volts ?; 0 – how many volts ? • 1 bit – How many nanoseconds ? • Bidirectional simultaneous transmission? • Electrical, mechanical, timing interfaces

37. Data Link layer • Turn the raw transmission into an error free communication line • Sets data in frames=thousands of bytes • Traffic regulation (flow control) • Access to the medium in broadcast shared communication lines

38. The Network Layer • Controls the operation of a subnet • How packets are routed from source to destination • Quality of service – congestion control • Fragmentation and inter-network problems

39. The Transport Layer • Accept data from upper layers and splits it into packets (small units) • Ensure that packets arrive correctly to the other end • Type of service: error free PtoP, preserve order or not, guarantees delivery or not, broadcast • True end-to-end layer

40. The Session Layer • Allows for establishing sessions • Session • Dialog control • Token management • Synchronization

41. The Presentation Layer • Syntax and semantics of data • Abstract data definitions/ encoding for information exchange between heterogeneous systems • Standard encoding “on the wire” • Exchange unit – record type

42. The Application Layer • Protocols needed by users: • HTTP - www • FTP – file exchange • TELNET – remote command • SSH – remote command • SMTP – mail exchange